U.S. application Ser. No. 844,783, filed Oct. 25, 1977 in the names of Bartholomew, Mach, and Wu, discloses the preparation of hydrated silver-containing glass articles which exhibit photosensitive characteristics. Glasses suitable for that invention consist essentially, on the anhydrous basis in mole percent, of about 3-25% Na.sub.2 O and/or K.sub.2 O and 50-95% SiO.sub.2, the sum of those components constituting at least 55% of the total composition, and about 0.1-3% total of a halide selected from the group of F.sup.-, Cl.sup.-, Br.sup.-, and I.sup.-. Such glasses are contacted either concurrently with the hydration step or subsequently thereto with an aqueous solution containing silver ions to cause an exchange of silver ions for sodium and/or potassium ions in the surface of the glass to occur accompanied with a reaction of silver ions with halide ions to form silver halide crystals.
Where the hydration and ion exchange are undertaken concurrently, the anhydrous glass is contacted with an aqueous silver ion-containing solution having a pH less than 4 at a temperature in excess of 100.degree. C. and at a pressure in excess of 20 psig for a sufficient length of time to hydrate at least a surface portion on the glass article and cause the replacement of sodium and/or potassium ions with silver ions in the hydrated glass.
Where the hydration and ion exchange are conducted sequentially, the anhydrous glass is initially exposed to a H.sub.2 O-containing gaseous environment having a relative humidity of at least 75% at a temperature of at least 100.degree. C. for a sufficient length of time to develop at least a surface portion on the article which is essentially saturated. Thereafter, the article is subjected to a gaseous environment wherein the relative humidity is less than 90% of that employed in the hydration step for a sufficient length of time to reduce the H.sub.2 O content at least within the surface portion but leaving an amount therein effective to impart thermoplastic properties thereto. Finally, the hydrated glass article is contacted with an aqueous silver ion-containing solution having a pH less than 5 at a temperature in excess of 100.degree. C. for a sufficient length of time to cause the replacement of sodium and/or potassium ions with silver ions in the hydrated glass.
The preferred glass composition regions were reported as consisting essentially, on the anhydrous basis in mole percent, of about 70-82% SiO.sub.2, 10-17% Na.sub.2 O and/or K.sub.2 O, 5-15% ZnO and/or PbO, 0.5-5% Al.sub.2 O.sub.3, and 0.5-3% halide. Cl.sup.- is the most desirable halide in amounts of about 0.1-3%. It was observed that 0.001-0.1 mole percent of a sensitizing agent such as CuO and/or CeO.sub.2 is advantageously included.
The products resulting from those inventive methods commonly exhibit a yellowish hue, the coloration becoming progressively darker as the silver ion concentration in the aqueous ion exchange solution is increased. Thus, the specification observes that the effect of utilizing greater silver ion concentrations in the ion exchange solution is to promote the extent of ion exchange reaction which leads to the development of a darker amber color.
The prior art relating to photosensitive and photochromic glasses has indicated that the quantity of silver must be held to a very low value to insure the formation of a transparent glass, as opposed to a translucent or opaque glass. Hence, for example, the silver content of the transparent photosensitive glasses of U.S. Pat. No. 2,515,936 will be held below 0.3% by weight, the silver content of the transparent photochromic glasses of U.S. Pat. No. 3,208,860 will be less than 0.7% by weight, and the silver content of the transparent photosensitive colored glasses of U.S. Pat. No. 4,017,318 will be maintained below 0.1%. In general, the maximum limits of allowable silver ion concentration in those transparent glasses, wherein the desired phenomenon is founded in the photosensitivity of silver, are a function of the redox state of the glasses, this circumstance being governed by oxygen ion activity which, in turn, is related to the quantities of thermoreducing and photoreducing agents in the glass. Thus, the amount of silver ion permissible in such transparent glasses is ordinarily not limited by the intrinsic solubility of silver ion in the base glass composition.
When the upper limit of silver ion concentration is approached in the glasses cited immediately above, the oxidation state of silver, i.e., the fraction of silver present as Ag.sup.+, becomes hypersensitive to the redox state of the glass. Consequently, the atmosphere of the melting tank, the melting temperature, impurities in the batch, e.g., organic contaminants from the sand, the temperature of heat treatment to develop silver halide crystals in photochromic glasses, and the temperatures of heat treatments to develop NaF-silver halide crystals in photosensitive colored glasses are well known to affect the behavior and physical properties of the final products. Moreover, in each case, the presence of any spontaneous warm-in silver may destroy or suppress the desired effects.